1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for supporting hybrid automatic repeat request (HARQ) in a wireless communication system.
2. Related Art
Wireless communication systems are widely spread all over the world to provide various types of communication services such as voice or data. In general, the wireless communication system is a multiple access system capable of supporting communication with multiple users by sharing available system resources (e.g., bandwidth, transmission power, etc.). Examples of the multiple access system include a code division multiple access (CDMA) system, a frequency division multiple access (FDMA) system, a time division multiple access (TDMA) system, an orthogonal frequency division multiple access (OFDMA) system, a single carrier frequency division multiple access (SC-FDMA) system, etc.
Current development in advanced wireless communication has led to the requirement of high spectral efficiency and reliable communication. Unfortunately, packet errors by fading channel environment and interferences originated from various sources make the capacity of overall system to be limited.
Hybrid automatic repeat request (HARQ) is an ARQ protocol combined with forward error correction (FEC) and is strongly considered as one of cutting edge technologies for future reliable communication. The HARQ scheme can largely be classified into two types. One is HARQ-chase combining (CC) which is disclosed in D. Chase, Code Combining: A maximum-likelihood decoding approach for combining an arbitrary number of noisy packets, IEEE Trans. on Commun., Vol. 33, pp. 593-607, May 1985. The other is HARQ-Increment Redundancy (IR). In the HARQ-CC, when a receiver detects an error through cyclic redundancy checking (CRC) while decoding the transmitted packet, the same packet with the same modulation and coding is retransmitted to the receiver. Meanwhile, in order to achieve a coding gain, the HARQ-IR retransmits different packets, in which parity bits can be manipulated through puncturing and repetition. To perform the HARQ, there is a need to exchange acknowledgement (ACK)/not-acknowledgement (NACK) information that indicates whether retransmission is necessary.
Adaptive modulation and coding (AMC) is also a technology for providing reliable communication. A base station (BS) determines a modulation and coding scheme (MCS) used for transmission by using a channel quality indicator (CQI) received from a user equipment (UE). In general, the CQI is an index of an entity of an MCS table showing a plurality of MCSs. The UE transmits the CQI by using two methods. One is that the CQI is transmitted periodically. The other is that the CQI is transmitted at the request of the BS.
3rd generation partnership project (3GPP) long term evolution (LTE) is a part of an evolved-universal mobile telecommunications system (E-UMTS) using evolved-universal terrestrial radio access (E-UTRA), and adopts the OFDMA in downlink and the SC-FDMA in uplink. Resource allocation of the 3GPP LTE is based on dynamic scheduling. A downlink physical channel of the 3GPP LTE can be divided into a physical downlink control channel (PDCCH) for carrying resource allocation information and a physical downlink shared channel (PDSCH) for carrying downlink data. An uplink physical channel can be divided into a physical uplink control channel (PUCCH) for carrying uplink control information and a physical uplink shared channel (PUSCH) for carrying uplink data. In downlink transmission, the UE first receives a downlink grant on the PDCCH, and then receives downlink data on the PDSCH indicated by the downlink grant. In uplink transmission, the UE receives an uplink grant on the PDCCH, and then transmits uplink data on the PUSCH indicated by the uplink grant. Dynamic scheduling is a method capable of effective resource allocation. However, the UE always has to receive the downlink/uplink grant first to transmit and/or receive data.
A signaling overhead is a major cause of low transmission efficiency and low frequency efficiency. In dynamic scheduling, in addition to reception of the PDCCH, the HARQ operation and the CQI transmission are carried out by using a plurality of signaling operations such as exchange of ACK/NACK information, exchange of a transmission parameter for the CQI, etc.
Accordingly, there is a need for a method capable of reducing a signaling overhead caused by CQI transmission in a process of performing HARQ.